Myotonic dystrophy type 1 (DM1), the most prevalent form of muscular dystrophy, is associated with progressive disability and premature death. Presently there are no treatments that reverse the muscle wasting and weakness in patients with DM1, or that slow the disease progression. This disorder is caused by an expansion of CTG repeats in the 3' untranslated region of the DMPK gene. Recent findings indicate that DM1 involves a novel RNA-mediated disease mechanism, initiated by the mutant DMPK mRNA, independently of DMPK protein. The transcripts from the mutant allele contain an expanded CUG repeat, and the pathogenic effects of the mutant RNA clearly derive from this poly(CUG)exp element. For example, we found that expression of poly(CUG)exp in the 3' untranslated region of an entirely different mRNA can recapitulate the disease in transgenic mice. Our collaborative studies have led to a multistep model for DM1 pathogenesis: (1) transcription of the mutant allele generates poly(CUG)exp RNA; (2) mutant transcripts accumulate in the nucleus in discrete foci (ribonuclear foci); (3) splicing factors in the muscleblind (MBNL) family, principally MBNL1, are sequestered in the ribonuclear foci; (4) reduced activity of MBNL1 protein leads to abnormal regulation of alternative splicing for a select group of pre-mRNAs, such as, the sarcolemmal chloride channel, ClC-1; and (5) expression of splice isoforms that are developmentally inappropriate leads to symptoms of DM1, such as, myotonia. In support of this model, we find that MBNL1 protein is recruited into ribonuclear foci so extensively that it is markedly depleted elsewhere in the nucleoplasm, and that splicing defects, or spliceopathy, in MBNL1 knockout mice are remarkably similar to those observed in DM1 patients. This model predicts that effects of DM1 in a particular nucleus are determined by levels of poly(CUG)exp accumulation in relation to supplies of MBNL1 protein, and that treatments which reduce the accumulation of poly(CUG)exp RNA or inhibit its interaction with MBNL1 have potential to stop disease progression and even accomplish a phenotypic reversal. The Aims of this proposal are to develop assays to screen for compounds that (1) reverse the spliceopathy in a cellular model of DM1; (2) inhibit the recognition of poly(CUG)exp RNA by MBNL1 protein in cells; or (3) inhibit the interaction of poly(CUG)exp RNA and MBNL1 protein in vitro. The proposed studies will be carried out in an environment that has experimental therapeutics of DM1 as its major focus. Thus, novel findings from a screening program could progress rapidly to preclinical testing in appropriate model systems and translation into therapeutic trials. Myotonic dystrophy, the most common form of muscular dystrophy in adults, causes progressive disability and premature death. Presently there are no treatments that improve the muscle weakness in people with myotonic dystrophy, or slow the disease progression. The goal of this project is to discover drugs that can be developed into effective treatments for myotonic dystrophy. [unreadable] [unreadable] [unreadable]